The present invention relates to automated inspection systems. More particularly, the present invention relates to methods and apparatus for inspecting portions of a substrate.
Automated visual inspection of substrates is jokingly said to be the technology of the “future.” That is, visual inspection technology that is something that is achievable only in the future, and not today. There are many reasons why reliable visual inspection of substrates have not been achievable today. Some of the reasons include that the high resolution sensor arrays that are often required have not been economically viable, and that acquisition of images that only highlight defects is virtually impossible.
As one example, in the field of display inspection systems, the displays that are to be inspected often have a large number of display pixels, such as 1900×1200, 1600×1200 or the like. Further, for the high resolution displays described above, if such displays are conventional RGB-type displays, each pixel includes as many as three color sub-pixels. Accordingly, whereas the display may have a stated 1600×1200 resolution, the inventors have determined that the effective inspection resolution for such displays is even higher. For example, a 1600×1200 display will actually have a horizontal resolution of 4800 sub-pixels, giving an effective resolution of at least 4800×1200. Accordingly, acquiring images of such displays for inspection purposes requires high resolution cameras.
In contrast, high resolution commercially available CCD cameras typically have resolutions such as 1900×1200, 4096×4096 (4K×4K), and the like. In light of this, the inventors have determined that the use of lower resolution cameras have advantages over the use of higher resolution cameras for inspection of such substrates. For example, high resolution cameras often have special operational requirements that are not needed for cameras having lower resolutions (e.g. 1K×1K or 2K×2K). Furthermore, lower resolution cameras are typically much cheaper than higher resolution cameras. For example it is believed that a 2K×2K sensor camera will cost less than one quarter the cost of a 4K×4K camera.
To further compound the problem, current displays, such as liquid crystal displays, typically have thin black borders surrounding each sub pixel. It is believed in the industry that the thin black border surrounding sub-pixels helps improve the contrast ratio of such displays and helps hide slight mis-alignments between adjacent display sub-pixels. These black borders are typically much smaller in width and height than the display sub-pixels, e.g. they are less than one-tenth the width of a display sub-pixel. In other embodiments, the relative sizes may be different.
FIG. 6 illustrates an exemplary display pixel to be inspected. As can be seen, one display pixel 600 is approximately square and currently range from 80 microns to 100 microns per side. As illustrated, display pixel 600 includes sub-pixels 610–630, typically representing red, green and blue display elements. These sub-pixels are separated by cell-walls that appear black on a display, hence the sub-pixels appear separated by thin black borders. For example, sub-pixels 610–630 are separated by cell-wall region 640–650 in FIG. 6. In this embodiment, the ratio of widths between a sub-pixel, such as sub-pixel 610 and a cell-wall region, such as 640 are approximately 10:1. A more detailed diagram illustrating the relationships between sub-pixels and cell-walls can be found in U.S. Pat. No. 5,754,678, assigned to the current assignee of the present patent application.
The inventors have determined that it is not necessary to inspect the thin black border surrounding such sub-pixels, but to only inspect the sub-pixels themselves. However, the inventors have learned that those thin black borders interfere with the inspection of the sub-pixels. More specifically, if two camera pixels capture each sub-pixel, as illustrated in FIG. 6, fulfilling the Nyquist sampling principle, there will still be aliasing during the capture of the thin black borders. As a result, the thin black borders will appear on a captured frame of the display as a Moiré artifact pattern. The inventors have discovered that this Moiré artifact pattern can be characterized as a low spatial frequency undulations or a “beat frequency” appearing horizontally and a typically different beat frequency appearing vertically on the image. The Moiré artifact, not considered a “defect” on the display, may also be quasi-repetitive.
Accordingly, when capturing images of a display for automatic inspection purposes, or the like, the inventors have discovered that this Moiré artifact on the captured image interferes with the inspection of the display. That is, one is unable to capture images of the display that highlight only true defects on the display such as bright pixels, dark pixels, and the like.
In light of the above, what is desired are methods and apparatus for capturing images of displays for inspection purposes without the drawbacks discussed above.